Therapies to prevent Alzheimer's Disease (AD) progression remain a high-unmet medical need. US Food and Drug Administration (FDA) approved acetylcholinesterase (AChE) inhibitor drugs, such as donepezil, rivastigamine and galantamine are indicated for symptomatic relief in persons with mild to moderate AD (Cummings J L, “Alzheimer's disease,” N Engl J Med (2004) 351, 56-67; Knowles J, “Donepezil in Alzheimer's disease: an evidence-based review of its impact on clinical and economic outcomes,” Core Evidence (2006) 1, 195-219). These drugs increase levels of available acetylcholine during synaptic transmission and thus compensate for the diminished function of cholinergic neurons. However, none of the drugs approved for AD are disease-modifying treatments that affect the underlying pathophysiology of the disease, so the duration of their benefit is short-term (Knowles, 2006). The development of successful disease-modifying treatments, in contrast, would have a long-term beneficial outcome on the course of AD progression.
The treatment of AD will require addressing the multiple triggers of pathogenesis. There are two primary neuropathologies in the brains of AD patients: i) extracellular protein plaques principally composed of Aβ peptides, also known as amyloid plaques; and ii) intracellular tangles of fibrils composed of tau protein found inside of neurons, also known as tau tangles. The advent and spread of neurotoxic oligomeric aggregates of Aβ is widely regarded as the key trigger leading to neuronal damage, which then leads to the accumulation of intracellular tau tangles, and finally to neuronal cell death in AD pathogenesis.
Beta-amyloid peptides (37 to 43 amino acids in length) are formed by sequential cleavage of the native amyloid precursor protein (APP) (Karran et al., “The amyloid cascade hypothesis for Alzheimer's disease: an appraisal for the development of therapeutics,” Nature Reviews (2011) 10, 698-712). Aberrant Aβ peptide isoforms that are 40 or 42 amino acids in length (Aβ 40 and 42) misfold into aggregates of oligomers that grow into fibrils that accumulate in the brain as amyloid plaques. More importantly for AD pathogenesis, the alternate fate of Aβ oligomers is to become trapped in neuronal synapses where they hamper synaptic transmission, which eventually results in neuronal degeneration and death (Haass et al., “Soluble protein oligomers in neurodegeneration: lessons from the Alzheimer's amyloid β-peptide,” Nature Reviews Mol. Cell Biol. (2007) 8:101-112; Hashimoto et al, “Apolipoprotein E, especially apolipoprotein E4, increases the oligomerization of amyloid beta Peptide,” J. Neurosci. (2012) 32, 15181-15192).
The cascade of Aβ oligomer-mediated neuronal intoxication is exacerbated by another AD trigger: chronic local inflammatory responses in the brain (Krstic et al., “Deciphering the mechanism underlying late-onset Alzheimer disease,” Nature Reviews Neurology (2013), Jan. 9 (1): 25-34). Alzheimer's disease has a chronic neuro-inflammatory component that is characterized by the presence of abundant microglial cells associated with amyloid plaque. (Heneka et al., “Acute treatment with the PPARγ agonist pioglitazone and ibuprofen reduces glial inflammation and Abeta1-42 levels in APPV717I transgenic mice,” Brain (2005) 128, 1442-1453; Imbimbo et al., “Are NSAIDs useful to treat Alzheimer's disease or mild cognitive impairment,” Front. Aging Neurosci (2010) 2 (article 19), 1-14). These cyclooxygenase (COX1/COX2)-expressing microglia, which phagocytose amyloid oligomers, become activated to secrete pro-inflammatory cytokines. (Hoozemans et al., “Soothing the inflamed brain: effect of non-steroidal anti-inflammatory drugs on Alzheimer's disease pathology,” CNS & Neurological Disorders—Drug Targets (2011) 10, 57-67; Griffin T S., “What causes Alzheimer's” The Scientist (2011) 25, 36-40; Krstic 2013). This neuro-inflammatory response, besides promoting local vascular leakage through the blood brain barrier (BBB). Zlokovic (Zlokovic B., “Neurovascular pathways to neurodegeneration in Alzheimer's disease and other disorders,” Nature Reviews Neurosci. (2011) 12, 723-738) has been implicated in driving further production of aberrant Aβ peptides 40 and 42 via modulation of gamma-secretase activity (Yan et al., “Anti-inflammatory drug therapy alters β-amyloid processing and deposition in an animal model of Alzheimer's disease,” J. Neurosci. (2003) 23, 7504-7509; Karran 2011) and to be detrimental to hippocampal neurogenesis in the adult brain (Gaparini et al., “Non-steroidal anti-inflammatory drugs (NSAIDs) in Alzheimer's disease: old and new mechanisms of action,” J. Neurochem (2004) 91, 521-536). Thus, neuro-inflammation, in combination with amyloid oligomer-mediated neuronal intoxication, creates a cycle that results in progressive neural dysfunction and neuronal cell death spreading throughout the brain in subjects with AD.
Compelling evidence from multiple epidemiology studies revealed that long-term dosing with non-steroidal anti-inflammatory drugs (NSAIDs) dramatically reduced AD risk in the elderly, including delayed disease onset, reduced symptomatic severity and slowed cognitive decline. (Veld et al., “Nonsteroidal anti-inflammatory drugs and the risk of Alzheimer's disease,” N. Engl. J. Med (2001) 345, 1515-1521; Etminan et al., “Effect of non-steroidal anti-inflammatory drugs on risk of Alzheimer's disease: systematic review and meta-analysis of observational studies,” Brit. Med. Journal (2003) 327, 1-5; Imbimbo, 2010). Three mechanisms have been proposed for how NSAIDs inhibit the processes that contribute to AD progression: i) by inhibiting COX activity to reduce or prevent microglial activation and cytokine production in the brain (Mackenzie, et al., “Nonsteroidal anti-inflammatory drug use and Alzheimer-type pathology in aging.” Neurology (1998) 50, 986-990; Alafuzoff et al., “Lower counts of astroglia and activated microglia in patients with Alzheimer's disease with regular use of non-steroidal anti-inflammatory drugs,” J. Alz. Dis. (2000) 2, 37-46; Yan, 2003; Gasparini, 2004; Imbimbo, 2010); ii) by reducing amyloid deposition (Weggen et al., “A subset of NSAIDs lower amyloidogenic Abeta42 independently of cyclooxygenase activity,” Nature (2001) 414, 212-216; Yan, 2003; Imbimbo, 2010); or iii) by blocking COX-mediated prostaglandin E2 responses in synapses (Kotilinek et al., “Cyclooxygenase-2 inhibition improves amyloid-β-mediated suppression of memory and synaptic plasticity,” Brain (2008) 131, 651-664.
Therefore, NSAIDs are predicted to dampen the neuro-inflammatory response and impact AD progression via several mechanisms. When administered together with drugs that inhibit Aβ oligomerization, the combination treatment paradigm is proposed to attenuate the multiple triggers leading to neurodegeneration and neuronal death. The decline in cognitive performance may be reversed, due to neuronal plasticity and neurogenesis in the hippocampus (Kohman et al., “Neurogenesis, inflammation and behavior,” Brain, Behavior, and Immunity (2013) 27, 22-32), if AD progression is arrested at a very early stage.